The physiological function and mode of regulation of transglutaminases are being studied as to their role in the formation of "provisional stroma" (fibrin or fibrin-connective tissue) during tissue or bone fracture repair, in the crosslinking of extracellular matrix proteins during cell-cell interaction, and in the modulation of specific cellular processes. The coagulant layer formed at injury sites is one of the vital elements of hemostasis and diathesis. The major constituent of this coagulant gel is fibrin. A number of proteins, e.g., fibronectin, thrombospondin, vitronectin, etc., that are known to be involved in cell attachment reactions are cross-linked to fibrin. It was found that fibrin(ogen) binds to specific cellular receptors of B16/F10 melanoma cells. These fibrinogen receptors are distinct from the well-characterized integrin super family of ECM receptors. Cellular transglutaminases in terminally differentiated epidermis catalyze the cross-linking of several proteins to form cornified envelope. The majority of cytosol granular layers.of epidermis appears to be activated during the apoptotic process in terminally differentiated epidermis to form corneum layers. A membrane-associated transglutaminase catalyzes the cross-linking of salivary proteins to oral epithelium to form the mucosal pellicle which serves as a protective barrier.